Super-sensitive interdigitated sensor for human-machine interaction

Masoumeh Hesam Mahmoudinezhad, Iain Anderson, Samuel Rosset
Biomimetics lab, Auckland bioengineering institute, University of Auckland, Auckland, New Zealand

We present a Tic-Tac-Toe game based on our soft and compliant compression sensor.
This new sensor design relies on interdigitated electrodes (IDEs) patterned on a PCB covered with a Carbon Black/ PDMS composite. The IDEs generate a fringing field that penetrates through the composite
when the voltage excitation applies, polarizing the dipoles of the composite. The capacitance of the IDEs sensor depends on the permittivity of the material on top of the electrodes, but not on its shape, and it therefore doesn’t matter that no deformation occur in the vicinity of the electrodes. The permittivity of the composite is very sensitive to any deformation, leading to large relative changes in capacitance when pressure is applied. External load causes a decrease in relative permittivity of the composite. This is due to a decrease in the contribution of interfacial dipoles such as MSW, filler, and charge dipoles. These dipoles are strain-dependent because the external load changes the filler network shape, decreasing the contribution of MSW dipoles on permittivity dramatically.
The features that make this sensor remarkable are:
1- The fabrication process is easy and cost-effective.
2- The sensor has a wide range of force detection from less than 1 N to over 90N.
3- The sensor shows great sensitivity to a low-pressure range (about 250% to 17KPa).
4- No compliant electrodes are required, and since standard PCB technology is used to manufacture electrodes, making arrays of sensors is trivial.
5- Skin-like structure: the sensor is as soft as human skin and agreeable to the touch: ideal for the surface of our tic-tac-toe.
6- It can be made thick to conform to object, making it a perfect sensor for a robotic gripper tasked with the manipulation of fragile object
Design of the Tic-Tac-Toe:
Our game demonstrates the sensor's sensitivity, and illustrates how this sensing technology can be used to map and measure compression forces. We divided the whole sensing area into nine zones. Each zone
contains an interdigitated electrode at the centre surrounded by 8 LEDs. At the top of the IDEs, a homogenous composite of CB/PDMS has been cast. All the wires are routed through the PCB and passed to the control box under the PCB. Capacitance read in parallel with a 10-channel Stretch Sense sensing board, which is connected to a Teensy development board via an SPI bus. We have used the addressable RGB LEDs. When the player pushes one of the playing squares, the capacitance changes. The control box detects the pressure and the LEDs around that sensor will light up with the colour of the active player.
This demonstration will be used for an upcoming outreach event with hundreds of school children. It has therefore been design to be large (210mm*200mm overall dimensions and 42mm*42mm dimensions of
each zone) and robust. The sensor has three modes:
1- player against player
2- player against the computer
3- the illumination of LEDs based on the intensity of touch